The present invention relates to the preparation of alkali metal percarbonates and more specifically to processes for producing sodium percarbonate by spraying.
Sodium percarbonate is frequently used in laundry washing and detergent compositions, bleaching agents, and in scouring powders. According to the prior art techniques, the percarbonate is produced by chilling a mixture of concentrated aqueous hydrogen peroxide and saturated sodium carbonate solution to precipitate the product. After draining and drying, a product having the formula Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2 and containing about 15.4 percent active oxygen is obtained.
With this process, the mother liquor of crystallization contains substantial active oxygen values and must be recycled for economic reasons. But such recycling cannot be completely effective because of the well-known instability of hydrogen peroxide, and in practice on the order of 15 to 20 percent of the active oxygen is lost. This instability is a result of the presence of the inevitable impurities, principally iron, in the commercial sodium carbonate starting material, and the recycling naturally increases the concentration of such impurities.
Incidentally, as mentioned above, sodium percarbonate is frequently included in laundry compositions, bleaching agents, and scouring powders, but the components of such cleaning agents, other than percarbonate, are very different, both in their form and their density, from particles of sodium percarbonate prepared by prior art process, and this makes it very difficult to obtain a homogeneous mixture.
It would be commercially desirable to be able to prepare sodium percarbonate by spraying. Various suggestions have been made along these lines, as in the French patent application of Degussa, No. 69/27886, filed Aug. 13, 1969. The claimed Degussa procedure contemplates spraying sodium percarbonate already prepared before it is introduced into the spray drying tower. This is, in fact, really a spray drying of the sodium percarbonate.